Mobile telephony networks were initially conceived for enabling voice communications, similarly to the wired, Public Switched Telephone Networks (PSTNs), but between mobile users. Mobile telephony networks have experienced an enormous spread, especially after the introduction of second-generation mobile cellular networks, and particularly digital mobile cellular networks such as those complying with the Global System for Mobile communications (GSM) standard (and its United States and Japanese corresponding systems).
The services offered by these cellular networks in addition to plain voice communications have rapidly increased in number and quality; just to cite a few examples, Short Messaging System (SMS) and Multimedia Messaging System (MMS) services, and Internet connectivity services have been made available in the last few years.
Similarly to the PSTNs, second-generation cellular networks are switched-circuit networks; this greatly limits the bandwidth that can be allocated for a given user, especially in second-generation mobile networks. On the contrary, data communications networks such as computer networks and, among them, the Internet, adopt packet switching schemes, which allow much higher data transfer rates.
Some solutions have been proposed to overcome the limitations of conventional, switched-circuit cellular networks such as the GSM networks, so as to enable users of mobile terminals efficiently exploiting services offered through the Internet. One of the solutions that is acquiring a significant popularity is the General Packet Radio Service (shortly, GPRS). The GPRS is a digital mobile phone technology compatible with GSM networks (actually, built on the existing GSM network architecture) that enables data transfer at a speed higher than that allowed by pure GSM. Essentially, the GPRS can be viewed as a GSM add-up that supports and enables packet-based data communication. Although third-generation wireless communications systems such as those complying with the Universal Mobile Telecommunication System (UMTS) are more promising in terms of data transfer rates, the GPRS is a ready-at-hand solution for enhancing the data exchange capabilities of already existing GSM networks.
In GPRS communications networks the information content is usually transferred in a point-to-point (p-t-p) modality (or unicast modality), upon activation of a session between a GPRS mobile phone (or mobile station) and a service provider connected to a packet data network, e.g. a server connected to the Internet; the activation of such a session involves the setting up of logic connections between the server and the GPRS mobile phone. In such a p-t-p communication mode, the radio resources to be allocated for the exchange of data between the ground GPRS network and the GPRS mobile stations depend on the number of different mobile stations simultaneously exploiting the GPRS services, even if the same GPRS service is being exploited by two or more mobile station users at the same time. Clearly, this limits the possibility of simultaneously accessing available GPRS services by several users, unless the radio resources are oversized.
Thus, it is desirable to have the possibility of delivering information contents related to a same GPRS service exploitable by two or more users at a time through a point-to-multipoint (p-t-M) modality, so as to save the amount of allocated resources.
In this respect, the 3GPP (3rd Generation Partnership Project) is discussing the implementation, in the GERAN (GSM/EDGE Radio Access Network) framework, of a new kind of service, named MBMS (Multimedia Broadcast/Multicast Service).
Basically, MBMS targets simultaneous distribution of information content (e.g. multimedia content) to more than one user from a single serving base station over a common radio resource.
A general problem related to a service exploiting a p-t-M modality, such as MBMS, is to provide a satisfying and reliable service at least for most users.
In this respect, a first approach followed by 3GPP was not to guarantee reception of MBMS at RAN (Radio Access Network) level: see 3GPP Technical Report TR 25.992 V6.0.0 (2003-09). According to this Technical Report, MBMS should not support individual retransmissions at the radio link layer, nor should it support retransmissions based on feedback from individual subscribers at the radio level.
In more recent submissions made at GERAN Meetings #18 and #19, some proposals have been made in order to provide the network with suitable procedures adapted to lowering the loss rate of the p-t-M transmission, or at least to derive information on the quality of service perceived by the users.
Document Tdoc G2-040286 (“Common Feedback Channel for MBMS delivery”), submitted by Siemens at the GERAN WG2 #18 bis Meeting, held in Phoenix, Ariz., U.S.A. from Mar. 22 to Mar. 26, 2004, discloses definition of a Common Feedback CHannel (CFCH) intended to be used as a feedback channel, where negative acknowledgments (nack) are sent as access bursts at precise times. More specifically, according to the proponents, feedback messages are sent by all interested Mobile Stations (MSs) as access bursts on the CFCH at a precise time: if a MS does not decode the RLC (Radio Link Control) block transmitted at time t, it will send an access burst at time t+Δt; if a MS successfully decodes the RLC block transmitted at time t, nothing is transmitted on the feedback channel at time t+Δt. The consequence is that, if an access burst is detected at time t+Δt, the network realizes that the block transmitted at time t has not been received (at least) by one MS.
Document Tdoc GP-040724 (“Draft CR to TS 43.246: Outer Coding in the RLC for MBMS”), submitted by Siemens at the GERAN #19 Meeting, held in Cancun, Mexico, from Apr. 19 to Apr. 23, 2004, discloses the use of outer coding using Reed-Solomon codes at the RLC layer for MBMS. According to this document, in the BSS (Base Station Subsystem) an outer coding unit is located between the LLC frame segmentation and the RLC/MAC block buffer, and before block headers (including the BSN, Block Sequence Number) are added to each LLC segment. For a code (n,k), the outer coding unit generates (n−k) parity blocks from k systematic blocks. The headers are then added to each segment sequentially starting with BSN=0. In the MS (Mobile Station), the received RLC/MAC blocks are passed, sequentially, to the outer coding unit together with the BSN. After decoding, the parity blocks are discarded and only the systematic blocks reassembled into LLC frames, which are then passed to the LLC layer.
Document Tdoc GP-041006 (“Draft CR to TS 43.246: User feedback using the CFCH”), submitted by Siemens at the GERAN #19 Meeting, held in Cancun, Mexico, from Apr. 19 to Apr. 23, 2004, discloses an outer coding and a feedback channel used together. In particular, the disclosed feedback channel is the CFCH (see above). In the document, it is disclosed that if outer coding at the RLC layer is used, it is not necessary to send a negative acknowledgement for every RLC block in error. According to the proposal, when outer coding and a feedback channel are used together, systematic and parity blocks are prepared at transmitting side. Then only the systematic ones are initially sent, while the parity ones are used to provide additional redundancy only once a negative acknowledgement is received.
In particular, for a code (n,k) the outer coding unit                generates n−k parity blocks from k systematic blocks        assigns BSNs to the n RLC blocks, with the following rule:                    systematic blocks will be assigned BSN=(M·n, . . . , M·n+k−1)            parity blocks will be assigned BSN=(M·n+k, . . . , M·n+n−1)where M is the outer code block number starting from zero.                        
Then only (M·n, . . . , M·n+k−1) blocks are initially transmitted, while parity ones are sent only upon feedback reception. Every time a negative acknowledgement—referred to the reception of the systematic blocks—is received, p parity blocks (M·n+k, . . . , M·n+k+p−1) are transmitted (where p is a divisor of n−k). If a further nack is received—referred to the first p parity blocks—other p parity blocks (M·n+k+p, . . . , M·n+k+2·p−1) are sent and so on, until no nack is received or all the n−k parity blocks have been sent.
At the receiving side, every MS will send a negative acknowledgement in the following cases:    1. every time the detection of the header of an RLC/MAC block (with its BSN) fails.    2. when, after detecting (from the BSN) the outer code block number M and outer code segment i (i=1, . . . , n), the MS computes the number of still expected blocks (LastM−i) and realizes that the following condition is true(number of already received RLC blocks)M+(LastM−i)<k             where LastM is the last expected outer code segment for outer code block number M, and is equal to k during the transmission of the k systematic blocks, to k+p during the transmission of the initial p parity blocks, to k+2·p . . . .        
Once a nack for block M is sent, a flag is internally set, and no other nack for the same outer code block can be sent during the transmission of the current set of blocks (k systematic, first p parity, second p parity ones, etc.).
Document Tdoc GP-040964 (“MBMS p-t-m channel with feedback”), submitted by Ericsson at the GERAN #19 Meeting, held in Cancun, Mexico, from Apr. 19 to Apr. 23, 2004, discloses a proposal for p-t-M with feedback. In such proposal, the MBMS service still uses unacknowledged transmission in the delivery of the MBMS data, but the network can poll for Packet Downlink Ack/Nack reports similar to the RLC acknowledged mode. In particular, the MSs are allocated a number of DL (Downlink) timeslots, e.g. 4. All MSs are allocated the same DL timeslots. The MSs are also allocated an UL (Uplink) timeslot. One of the DL timeslots is the “main” timeslot on which polling requests are sent. MSs are polled for Ack/Nack reports with their unique MS ID. The MS should receive blocks that have a valid MS ID in the TFI (Temporary Flow Identifier) field (a subset of the TFI: e.g. those match 0xxxx). The MBMS MS will first look at the first bit of the TFI. If that is set to 1, the MBMS MS will ignore the block. If it is set to 0, it will receive the block. The MS will also look at the four last bits of the TFI to see if it matches the MS's MS ID. If so, it will send an ACK/NACK report in the UL block indicated by the RRBP.
Document Tdoc GP-040877 (“Proposal for MS counting, addressing and RLC/MAC Ack/Nack management in MBMS”), submitted by Telecom Italia at the GERAN #19 Meeting, held in Cancun, Mexico, from Apr. 19 to Apr. 23, 2004, discloses methods for counting, addressing the MSs and managing the RLC/MAC Ack/Nack in MBMS. According to the proposal, in order to guarantee an adequate perception of the service from the users' point of view, the RLC/MAC mode of operation has to be an acknowledged one. Since all the MSs involved in the specified MBMS are multiplexed on the same timeslots on the DL and addressed via the same TFI, a further identifier for addressing a specific MS is needed, named MFI (Mobile Flow Identity), a new field to be included, when needed, in the extended RLC header of an RLC/MAC block. The global (TFI+MFI) identifier allows the BSS to address a specific MS among those involved in the specified MBMS session. A MS addressed via (TFI+MFI) sends a Packet Downlink Ack/Nack, including its MFI, in order to let the BSS detect the correct identity of the responding MS. The BSS processes all the Packet Downlink Ack/Nack messages received within a request period from all the MSs, in order to manage retransmissions of data blocks. The retransmissions on the BSS side may be performed according to an exhaustive algorithm, in which all the radio blocks referred to as Nacked in any received Packet Downlink Ack/Nack are retransmitted, or to a selective algorithm, based on the overall number of Nacked radio blocks, on a threshold relevant to the percentage of MSs requesting the retransmission of a specific radio block and on the MFI of the MSs requesting the retransmissions.